The invention relates to a method for modulating an optical signal and an optical transmitter for performing the method.
Several methods for modulating optical transmission signals are known in the literature. One of the best-known is the nonreturn-to-zero (NRZ) modulation technique represented in FIG. 2. In the NRZ method each logical bit (logical “1”-pulse) is transmitted with a pulse width equal to the full bit period T=1/B, where B is the bit-rate at which the pulses are to be transmitted (in bit/s).
FIG. 2a shows an diagram of the intensity (solid line) and phase (dashed line) of a typical NRZ-modulated optical signal of a bit signal of 16 consecutive bits with three pairs of consecutive “1”-bit pulses. The intensity of the optical signal between the two consecutive “1”-pulses of each pair remains constant and does not return to zero. FIG. 2b shows the optical spectrum of the NRZ optical signal of FIG. 2a. 
FIG. 2c represents a schematic of the most conventional way of creating a NRZ optical signal: A laser source 1 generates a continuous wave optical signal (carrier signal) which is modulated by an electrical non-return to zero bit signal with bit-rate B in a following first Mach-Zehnder modulator 2. The modulator 2 converts the electrical bit signal into an intensity modulation of the optical signal, so that an optical output signal of the NRZ type is generated.
FIG. 3 shows the return-to-zero (RZ) modulation method as an alternative possibility for modulating an optical signal. In the RZ method, the intensity of the optical signal between two subsequent “1”-pulses returns to zero, cf. FIG. 3a. Consequently, the pulse width is no longer equal to the full bit period T. The bandwidth of the optical spectrum of the RZ optical signal, represented in FIG. 3b, is broader than that of the NRZ optical signal of FIG. 2b. The most conventional way of generating an RZ signal is shown in FIG. 3c. First, a NRZ signal is generated in the way shown in FIG. 2c and then, the NRZ signal enters a second Mach-Zehnder modulator 3 to which a sinusoidal electrical signal is applied, thus generating a sinusoidal intensity modulation of the NRZ signal with an information frequency (in Hz) corresponding to the bit-rate B (in bit/s). As a result, the NRZ input signal to the second modulator 3 is converted into a RZ output signal.
At 40 Gb/s channel rate, numerous studies has shown that departing from the conventional modulation techniques described above, relying only on intensity modulation, is a powerful means to contain propagation impairments, and hence increase system margins. Among these impairments, intra-channel nonlinear effects are admittedly the most penalizing at 40 Gb/s.
When passed into a fiber link (inherently dispersive), any optical pulse within a given wavelength division multiplexing (WDM) channel is broadened and, should it be surrounded by neighboring pulses, tends to overlap them. As a result, pattern-dependent interactions occur. These interactions cannot be fully undone by pulse compression into a dispersion-compensating fiber, because they are nonlinear. They are usually referred to as nonlinear intrachannel effects.
Several modulation formats have been praised for their superior resistance to nonlinear effects when compared to conventional intensity modulated Return-to-Zero (RZ) and Non-Return-to-Zero (NRZ). One basic solution against intra-channel effects is to contain pulse broading due to dispersion, which can be obtained by combining intensity and phase modulation.
Carrier-Suppressed RZ (CSRZ), represented in FIG. 4, was proposed for that purpose. In this format, the phase of every bit of an RZ signal is rotated by π, see FIG. 4a (dashed line). The optical spectrum of the CSRZ signal is represented in FIG. 4b. 
A conventional apparatus for generating a CSRZ signal is shown in FIG. 4c, similar to the one shown in FIG. 3c. In contrast to FIG. 3c, the sinusoidal signal driving the modulator is at half the information frequency, so that both frequency and phase of an NRZ input signal from the first modulator 2 are changed. Besides, the second Mach-Zehnder modulator 3 is preferably a dual-arm modulator, but not necessarily. In the two arm configuration, the same sinusoidal signal with half the frequency corresponding to the bit-rate B is applied to both arms. Though better than RZ, the CSRZ scheme is not very effective against intrachannel nonlinear effects.
Another solution consists in applying a sinusoidal phase to an RZ signal, to make chirped RZ (CRZ), but CRZ comes with an increased channel spectral bandwidth beyond what is acceptable for 40 Gb/s WDM applications.
Yet another method for the mitigation of intrachannel nonlinear effects is pair-wise alternate phase RZ (PAPRZ), represented in FIG. 5, which is basically similar to CSRZ, but with a π-phase rotation every second bit rather than every bit, see FIG. 5a for the phase rotation and FIG. 5b for the optical spectrum. A conventional apparatus for generating a PAPRZ signal is shown in FIG. 5c. The scheme includes the setup for generating an RZ signal shown in FIG. 3c, followed by a third modulator 4 for generating a phase-shift of every second bit of the RZ signal, to which a square-like clock signal with a frequency equal to one fourth of the information frequency B is applied. The PAPRZ scheme is much more efficient against intra-channel effects than CSRZ but has the drawback that three modulators 2, 3, 4 are required.
Other approaches for rotating the phase of the optical signal in a more random (pattern-dependent) manner exist, e.g. the differential-phase shift keying (DPSK) family, namely Return-to-Zero DPSK or NRZ-DPSK. The drawback of the DPSK family modulation techniques is that an electrical pre-coder, a temperature-stabilized Mach-Zehnder differential decoder and a balanced receiver are required. Another method of pattern-dependent phase shifting is the so-called phase shaped binary transmission (PSBT), for the application of which an electrical pre-coder and a careful control of the RF signal chain are required.